In a digital communication system, it is preferable to use constant amplitude modulations, for example continuous phase modulations, because the latter allow the range of the transmitted signal to be maximized. Specifically, constant envelope modulations have the advantage of allowing the emitted signal to have an almost constant power. The continuity of the phase allows the signal to occupy a narrower bandwidth, and the constant envelope of the signal allows nonlinearities in the transmission channel to be better withstood and the amplifiers of the system to function closer to their saturation point.
Thus, to decrease constraints on the spectral width of the transmitted signal, notably its occupied frequency bandwidth, and the disruption potentially caused to adjacent channels, it is known to use continuous phase modulation methods such as Gaussian minimum shift keying (GMSK) modulation.
Continuous phase modulations, although effective, have the drawback of introducing substantial interference between symbols in the received signal. The symbol received at the instant t is distorted at least by its neighbors emitted at the instants t−1 and t+1, thereby making the decision that a receiver must make to determine the initial state of said symbol more difficult. For example, in the case of a binary modulation, a symbol may take eight different states. The receiver will therefore have to make a decision among these eight possibilities in order to determine the corresponding bit. Receiver complexity and the disruption caused by interference between symbols increases as the number of possible symbol states increases. In addition, prior-art methods for demodulating transmitted continuous phase modulated signals have the drawback of being complex because they often use an equalizer and a maximum likelihood sequence estimator (MLSE) implemented, for example, via a Viterbi algorithm.
The French patent application of the Applicant, published under the number FR 2 938 988, describes a continuous phase modulation method that allows interference between symbols on reception to be decreased.
The method described in this application is based on implementation on emission of a linear combination of these symbols to be emitted before they are filtered.
Although the aforementioned method improves the level of inter-symbol interference relative to a conventional continuous phase modulation, it does not allow this effect to be completely suppressed.
There is therefore a need for an improved continuous phase modulation method that allows inter-symbol interference to be almost completely prevented.